1. Field of the Invention
The present invention relates to a power supply apparatus of a high-frequency inverter type and, more particularly, to a power supply apparatus for generating a low-noise output voltage which can stably light, e.g., a discharge lamp.
2. Description of the Related Art
A power supply apparatus shown in FIG. 1 is conventionally known. In this power supply apparatus, an AC power source 1 is connected to the input portion of a full-wave rectifier 3 through a filter 2. The filter 2 is constituted by an inductor and a capacitor. The full-wave rectifier 3 is constituted by a diode bridge. The output portion of the full-wave rectifier 3 is connected to a switching transistor 5 through an inductor 4, and is further connected to a smoothing capacitor 7 through a forward-biased diode 6. The smoothing capacitor 7 is connected in parallel with a series circuit of a primary winding 12a of a transformer 12 and a switching transistor 13. A high-frequency inverter 8 includes this series circuit, a resonance capacitor 14 connected in parallel with the primary winding 12a, a diode 15 connected in parallel with the switching transistor 13, and a secondary winding 12b of the transformer 12 serving as an output portion of the inverter 8. One end of the secondary winding 12b is connected to one end of a filament 10a of a discharge lamp 10, while its the other end is connected to one end of a filament 10b of the discharge lamp 10 through a capacitor 9. A starting capacitor 11 is connected between the other ends of the filaments 10a and 10b.
In the power supply apparatus, the switching transistor 5 performs a high-frequency switching operation under the control of a control circuit 16. When the switching transistor 5 is turned on, a current flows in the inductor 4 in the presence of a pulsating voltage output from the full-wave rectifier 3. Owing to this current, energy stored in the inductor 4 is superposed on the pulsating voltage when the switching transistor 5 is turned off, thus charging the smoothing capacitor 7 through the diode 6. When such a switching operation of the switching transistor 5 is repeated, the charged voltage across the smoothing capacitor 7 is smoothed. Meanwhile, a switching current flows in the inductor 4 such that an envelope corresponding to the pulsating voltage appears as a peak. This switching current becomes a sinusoidal current in phase with the AC voltage from the AC power source 1 through the filter 2. Therefore, the input current basically contains no harmonic component, and a high power factor can be obtained.
The charged voltage across the smoothing capacitor 7 is supplied to the high-frequency inverter 8 as power source voltage, and the high-frequency switching of the switching transistor is performed in the presence of this power source voltage to operate an oscillation circuit constituted by the primary winding 12a of the transformer 12 and the resonance capacitor 14. A resonant voltage from the oscillation circuit is transmitted to the secondary winding 12b of the transformer 12. An output voltage from the secondary winding 12b is applied to the discharge lamp 10. When a current flows in a series circuit of the starting capacitor 11 and filaments 10a and 10b upon application of the output voltage, the filaments 10a and 10b are pre-heated, and at the same time the voltage across the starting capacitor 11 is increased. The discharge lamp 10 is lighted when a high voltage is generated across the starting capacitor 11 and applied to the filaments 10a and 10b.
Since the power supply apparatus is operated by means of so-called chopper control, very few harmonic components are contained in the input current to this apparatus. However, a DC current corresponding to a loss in a load circuit made up of the high frequency inverter 8 and discharge lamp 10 continuously flows from the full-wave rectifier 3 to the load circuit throughout the entire period during which the pulsating voltage is output from the full-wave rectifier 3. For this reason, each circuit component needs to have a large capacity to process a large quantity of energy. Such a circuit component is large in size and expensive. In addition, the switching transistor 5 must be periodically switched so as not to leave energy stored in the inductor 4 in the condition where the instantaneous value of the pulsating voltage is constantly changing. For such control, the control circuit 16 is inevitably complicated in structure and increased in size. With regard to energy processing by a switching operation of the switching transistor 5, since this operation is continuously performed throughout the entire period during the pulsating voltage is applied to the switching transistor 5, a large switching loss is caused. Furthermore, since a switching current has a triangular waveform, the amount of noise produced is increased. Moreover, the capacity of each circuit component connected to the power lines must be increased to cope with an excessive inrush current flowing into the smoothing capacitor 7 when the power is turned on. This also contributes to an increase in the size of the power supply apparatus.